1. Field of the Invention
This invention relates to antenna diversity receivers, particularly those suitable for use for wideband radio reception and more particularly for multi-carrier systems.
2. Description of the Prior Art
Antenna diversity receivers use multiple antennas to overcome signal quality degradation caused by multipath fading. If the antennas are arranged such that their outputs fade independently, then the signals from the antennas can be combined to produce a signal with higher quality since it is unlikely that both antennas (branches) will simultaneously be in a deep fade. This allows the receiver to be used in areas with lower signal strengths or to provide higher signal quality and reliability within the normal system coverage area.
A common form of diversity combiner is a switch combiner, in which only one complete receiver is needed. The receiver is switched between the antennas and makes a judgment as to which antenna provides the strongest signal. Numerous schemes for doing this exist, but it is believed that none of them address suitable strategies for wideband channels. In all cases, switch combining performs less well than selection combining, in which two receivers are available so that the performance of both antennas can be simultaneously monitored, but a switch is used to select the signal from only one of them at a time. Maximal ratio combining (MRC) involves using, simultaneously, a plurality of receivers each operating on a signal from a respective antenna, and using signal processing to combine the outputs of the receivers. This gives better performance than either switch combining or selection combining, but is somewhat more expensive.
In a wideband fading channel, the bandwidth of the transmitted signal is wider than the coherence bandwidth of the channel (see S. R. Saunders, “Antennas and Propagation for Wireless Communication Systems”, John Wiley & Sons, ISBN 0471986097, July 1999, for precise definitions). This implies that different parts of the received signal bandwidth will be faded to different extents, so the choice of the best antenna is not clear. A conventional switch combiner could make a decision based on the total power available over the whole signal bandwidth, by performing a vector sum of the respective channel outputs of the receiver filter. FIG. 1 shows (curve A) that this yields only minor diversity gain when the delay spread is large, i.e. when there are significant delayed versions of the signal arriving at the receiver due to multipath echoes. The simulations in this figure assume two identical vertical antennas, with a mobile speed of 20 km/hr. The system simulated represents the ITU-T ISDB-T digital broadcasting standard. Diversity gain is referenced to the power required to achieve a bit error rate of 2×10−4. Curve B shows that the results when selection combining is used instead of switch combining are not significantly better.
Choosing a single antenna, based on whichever criteria, and using this for the reception of the whole ISDB-T bandwidth can lead to significant degradation in performance. Mostly, this will be due to the fact that somewhere within the signal bandwidth there will be a deep null, so although at some carriers within the bandwidth there may be excellent diversity gain, there is none achieved at other carriers, with the resultant diversity gain essentially an average across the bandwidth.
Given that delay spread has been shown to produce this significant performance degradation, it would be attractive to have a combining technique which avoids this problem, but without the expense of MRC systems, and preferably using only one receiver.
Accordingly, it would be desirable to provide a switch diversity combiner which has improved performance in high delay-spread environments.